gmtime(3C)
NAME
ctime, ctime_r, localtime, localtime_r, gmtime, gmtime_r,
asctime, asctime_r, tzset - convert date and time to string
SYNOPSIS
#include <time.h>
char *ctime(const time_t *clock);
struct tm *localtime(const time_t *clock);
struct tm *gmtime(const time_t *clock);
char *asctime(const struct tm *tm);
extern time_t timezone, altzone;
extern int daylight;
extern char *tzname[2];
void tzset(void);
char *ctime_r(const time_t *clock, char *buf, int buflen);
struct tm *localtime_r(const time_t *clock, struct tm *res);
struct tm *gmtime_r(const time_t *clock, struct tm *res);
char *asctime_r(const struct tm *tm, char *buf, int buflen);
POSIX
cc [ flag... ] file... -D_POSIX_PTHREAD_SEMANTICS [ library... ]
char *ctime_r(const time_t *clock, char *buf);
char *asctime_r(const struct tm *tm, char *buf);
DESCRIPTION
The ctime() function converts the time pointed to by clock,
representing the time in seconds since the Epoch (00:00:00
UTC, January 1, 1970), to local time in the form of a 26-
character string, as shown below. Time zone and daylight
savings corrections are made before string generation. The
fields are in constant width:
Fri Sep 13 00:00:00 1986\n\0
The ctime() function is equivalent to:
asctime(localtime(clock))
The ctime(), asctime(), gmtime(), and localtime() functions
return values in one of two static objects: a broken-down
time structure and an array of char. Execution of any of the
functions can overwrite the information returned in either
of these objects by any of the other functions.
The ctime_r() function has the same functionality as ctime()
except that the caller must supply a buffer buf with length
buflen to store the result; buf must be at least 26 bytes.
The POSIX ctime_r() function does not take a buflen parame-
ter.
The localtime() and gmtime() functions return pointers to tm
structures (see below). The localtime() function corrects
for the main time zone and possible alternate ("daylight
savings") time zone; the gmtime() function converts directly
to Coordinated Universal Time (UTC), which is what the UNIX
system uses internally.
The localtime_r() and gmtime_r() functions have the same
functionality as localtime() and gmtime() respectively,
except that the caller must supply a buffer res to store the
result.
The asctime() function converts a tm structure to a 26-
character string, as shown in the previous example, and
returns a pointer to the string.
The asctime_r() function has the same functionality as asc-
time() except that the caller must supply a buffer buf with
length buflen for the result to be stored. The buf argument
must be at least 26 bytes. The POSIX asctime_r() function
does not take a buflen parameter. The asctime_r() function
returns a pointer to buf upon success. In case of failure,
NULL is returned and errno is set.
Declarations of all the functions and externals, and the tm
structure, are in the <time.h> header. The members of the tm
structure are:
int tm_sec; /* seconds after the minute - [0, 61] */
/* for leap seconds */
int tm_min; /* minutes after the hour - [0, 59] */
int tm_hour; /* hour since midnight - [0, 23] */
int tm_mday; /* day of the month - [1, 31] */
int tm_mon; /* months since January - [0, 11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday - [0, 6] */
int tm_yday; /* days since January 1 - [0, 365] */
int tm_isdst; /* flag for alternate daylight savings time */
The value of tm_isdst is positive if daylight savings time
is in effect, zero if daylight savings time is not in
effect, and negative if the information is not available.
Previously, the value of tm_isdst was defined as non-zero if
daylight savings was in effect.
The external time_t variable altzone contains the differ-
ence, in seconds, between Coordinated Universal Time and the
alternate time zone. The external variable timezone contains
the difference, in seconds, between UTC and local standard
time. The external variable daylight indicates whether time
should reflect daylight savings time. Both timezone and
altzone default to 0 (UTC). The external variable daylight
is non-zero if an alternate time zone exists. The time zone
names are contained in the external variable tzname, which
by default is set to:
char *tzname[2] = { "GMT", "" };
These functions know about the peculiarities of this conver-
sion for various time periods for the U.S. (specifically,
the years 1974, 1975, and 1987). They start handling the new
daylight savings time starting with the first Sunday in
April, 1987.
The tzset() function uses the contents of the environment
variable TZ to override the value of the different external
variables. It is called by asctime() and can also be called
by the user. See environ(5) for a description of the TZ
environment variable.
Starting and ending times are relative to the current local
time zone. If the alternate time zone start and end dates
and the time are not provided, the days for the United
States that year will be used and the time will be 2 AM. If
the start and end dates are provided but the time is not
provided, the time will be 2 AM. The effects of tzset()
change the values of the external variables timezone,
altzone, daylight, and tzname.
Note that in most installations, TZ is set to the correct
value by default when the user logs on, using the local
/etc/default/init file (see TIMEZONE(4)).
ERRORS
The ctime_r() and asctime_r() functions will fail if:
ERANGE
The length of the buffer supplied by the caller is not
large enough to store the result.
USAGE
These functions do not support localized date and time for-
mats. The strftime(3C) function can be used when localiza-
tion is required.
The localtime(), localtime_r(), gmtime(), gmtime_r(),
ctime(), and ctime_r() functions assume Gregorian dates.
Times before the adoption of the Gregorian calendar will not
match historial records.
EXAMPLES
Example 1: Examples of the tzset() function.
The tzset() function scans the contents of the environment
variable and assigns the different fields to the respective
variable. For example, the most complete setting for New
Jersey in 1986 could be:
EST5EDT4,116/2:00:00,298/2:00:00
or simply
EST5EDT
An example of a southern hemisphere setting such as the Cook
Islands could be
KDT9:30KST10:00,63/5:00,302/20:00
In the longer version of the New Jersey example of TZ,
tzname[0] is EST, timezone is set to 5*60*60, tzname[1] is
EDT, altzone is set to 4*60*60, the starting date of the
alternate time zone is the 117th day at 2 AM, the ending
date of the alternate time zone is the 299th day at 2 AM
(using zero-based Julian days), and daylight is set posi-
tive. Starting and ending times are relative to the current
local time zone. If the alternate time zone start and end
dates and the time are not provided, the days for the United
States that year will be used and the time will be 2 AM. If
the start and end dates are provided but the time is not
provided, the time will be 2 AM. The effects of tzset() are
thus to change the values of the external variables
timezone, altzone, daylight, and tzname. The ctime(),
localtime(), mktime(), and strftime() functions also update
these external variables as if they had called tzset() at
the time specified by the time_t or struct tm value that
they are converting.
BUGS
The zoneinfo timezone data files do not transition past Tue
Jan 19 03:14:07 2038 UTC. Therefore for 64-bit applications
using zoneinfo timezones, calculations beyond this date
might not use the correct offset from standard time, and
could return incorrect values. This affects the 64-bit ver-
sion of localtime(), localtime_r(), ctime(), and ctime_r().
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | MT-Safe with exceptions |
|_____________________________|_____________________________|
| CSI | Enabled |
|_____________________________|_____________________________|
SEE ALSO
time(2), Intro(3), getenv(3C), mktime(3C), printf(3C),
putenv(3C), setlocale(3C), strftime(3C), TIMEZONE(4), attri-
butes(5), environ(5)
NOTES
When compiling multithreaded programs, see Intro(3), Notes
On Multithreaded Applications.
The return values for ctime(), localtime(), and gmtime()
point to static data whose content is overwritten by each
call.
Setting the time during the interval of change from timezone
to altzone or vice versa can produce unpredictable results.
The system administrator must change the Julian start and
end days annually.
The asctime(), ctime(), gmtime(), and localtime() functions
are unsafe in multithread applications. The asctime_r() and
gmtime_r() functions are MT-Safe. The ctime_r(),
localtime_r(), and tzset() functions are MT-Safe in mul-
tithread applications, as long as no user-defined function
directly modifies one of the following variables: timezone,
altzone, daylight, and tzname. These four variables are not
MT-Safe to access. They are modified by the tzset() function
in an MT-Safe manner. The mktime(), localtime_r(), and
ctime_r() functions call tzset().
Solaris 2.4 and earlier releases provided definitions of the
ctime_r(), localtime_r(), gmtime_r(), and asctime_r() func-
tions as specified in POSIX.1c Draft 6. The final POSIX.1c
standard changed the interface for ctime_r() and
asctime_r(). Support for the Draft 6 interface is provided
for compatibility only and might not be supported in future
releases. New applications and libraries should use the
POSIX standard interface.
For POSIX.1c-compliant applications, the
_POSIX_PTHREAD_SEMANTICS and _REENTRANT flags are automati-
cally turned on by defining the _POSIX_C_SOURCE flag with a
value >= 199506L.
Man(1) output converted with
man2html